1. Field of the Invention
The present invention relates to the field of process control valves. More particularly, the present invention provides an improved pressure regulating valve and control system especially for use in laboratory process systems.
2. Description of Related Art
In a wide variety of process control systems it is often desirable to carefully regulate the pressure in a portion of the process. For example, in a laboratory apparatus it is often necessary to accurately regulate liquid or gas pressure in a vessel. This is usually accomplished by installing a backpressure control valve on the outlet of the vessel. The pressure upstream of the valve is monitored and the valve is opened if the upstream pressure becomes too high. Conversely, if the pressure becomes too low, the valve is closed. Upstream pressure control valves operate in reverse and are installed upstream of the vessel. Downstream pressure is monitored and the valve is closed when the pressure becomes too high. The valve is opened as downstream pressure becomes too low.
Two types of valves are commonly used as backpressure control valves. In spring-loaded valves, the pressure of the fluid or gas being controlled is exerted against a spring in the valve. As the pressure of the fluid being controlled increases, the spring is compressed, resulting in the valve being closed. As the pressure of the fluid being controlled decreases, the spring overcomes pressure exerted by the fluid and the valve opens. The pressure at which the valve regulates pressure is generally adjusted by way of a threaded cap on the spring. Tightening the cap compresses the spring and, therefore, increases backpressure.
Dome-loaded regulators utilize a diaphragm plate on which pressure from a dome is exerted. As the system pressure increases, the diaphragm pressure is overcome, and liquid or gas bleeds out through a vent hole, permitting the valve to open. Dome pressure is controlled by using another pressure regulator in line with the dome regulator.
Both of the above-described pressure control valves have a number of limitations. For example, such valves often utilize a compressible element between the control mechanism and the element which restricts flow. Therefore, slow response and long term drift are often observed in such valves. Valve behavior also varies with the type of fluid in the system, which can lead to serious perturbations in the operation of a system. Further, both types of valves often require operator attention to maintain accuracy over long time intervals. Still further, the above-described valves result in difficulties in automating process control because they require elaborate motor-driven mechanisms (for the spring-loaded valves) or sophisticated servo-controlled gas or liquid control valves (for the dome-loaded valves). Consequently, present process control valves permit only crude control over the pressure in a process, even with computer controlled systems.
It is seen that an improved pressure control method and apparatus is desired.